1. Field of the Invention
This invention relates to a surface undulation inspection apparatus which detects an undulatory defect having a small profile change among defects generated in glass, a metal sheet, a coated surface and the like.
2. Description of the Prior Art
The inspection of a surface defect of a sheet material has been conducted by an apparatus for projecting a laser light on a specimen surface to receive a reflecting light, or a system for uniformly illuminating a specimen surface and photographing the specimen surface by a television camera, a line sensor or the like. However, in this apparatus or system, when a surface profile of a defect becomes small, the intensity and change in position of the reflecting light are very small, rendering detection impossible.
On the other hand, as an apparatus for measuring a surface flatness of a surface roughening of a coated surface, there is a "Flatness Measuring Apparatus" disclosed in Japanese Patent Laid Open Publication No. 76,004/1981. FIG. 1 shows the structure of a conventional flatness measuring apparatus disclosed in said patent publication, in which numeral 25 denotes a light source, 26 denotes a lattice pattern, 31 denotes a projection lens, 41 denotes a specimen to be inspected, 27 denotes a focussed lattice pattern image, and 61 denotes a photoelectric conversion element.
The operation of the conventional apparatus will be described below. The lattice pattern 26 illuminated by the light source 25 is projected by the projection lens 31. When the specimen to be inspected 41 is inserted in the midst of a projection channel, the projection channel is bent by the specimen to be inspected 41. The photoelectric conversion element 61 is moved on the lattice pattern image 27 focussed at that time to electrically read the lattice pattern image. In such a structure as just mentioned, when an abnormal flatness such as a distortion occurs in the specimen to be inspected, a distortion occurs in the lattice pattern image 27 being focussed and a pattern pitch changes. This is detected by the photoelectric conversion element 61, which is statistically compared with an average normal pitch to thereby arithmetically calculate a flatness within a view of inspection.
Since the conventional flatness measuring apparatus is constructed as described above, the view of inspection is determined by an installed position of the specimen to be inspected within a pattern projection system and is measured in mm. Because of this, the efficiency for inspecting a surface having a wide area is very poor. In a case of a reduced projection system (in the aforesaid publication, magnification .beta. is less than 1), a large aperture projection lens is required in order to widen the view of inspection. For example, in order that a specimen is arranged at a position one half (1/2) of a projection distance to realize the view of inspection having a diameter of 50 cm, an aperture of a lens is necessary to have 1 m or more, which is unrealistic. Further, in order that a specimen is arranged at a position one half (1/2) of a projection distance in an enlarged projection system to realize the view of inspection having a diameter of 50 cm, the moving scanning distance of the photoelectric element 61 must also be 1 m, and a burden of the mechanism of the system increases in terms of the construction of a system. Moreover, the conventional flatness measuring apparatus has no object to detect an average distortion within a view of field to detect a local distortion. Therefore, the conventional apparatus statistically processes the change of pitch within a view of field, failing to detect a local defect such as a small undulation, most of which occurs in a normal surface. Further, since light passing through a local portion of the specimen surface is not optically specified, an optical abnormality resulting from a small undulation becomes embedded in the reflected light from a major portion of normal surface, possibly rendering detection impossible. Thus, the conventional apparatus has many problems in practical use and principle for the surface inspection.
FIG. 2 is a structural view of a conventional surface undulation inspection apparatus shown, for example, in the specification and drawings attached to Japanese Patent Application No. 175140/1991. In the drawings, numeral 1 denotes a patterning light source for generating a desired light source pattern, and numeral 2 denotes a specimen to be inspected for any undulatory defects. Numeral 3 denotes a camera which is arranged opposite to the patterning light source 1 and which encloses a pin hole to selectively utilize only a principal ray for focussing.
The operation of the above disclosure will be described. The patterning light source 1 is opposed to the camera 3 having a pin hole enclosed therein with the specimen to be inspected 2 sandwiched therebetween, and a focal position of the camera 3 is adjusted to the patterning light source 1 via the specimen to be inspected 2. At that time, the light contributed to the image pickup of the camera 3 is selected in the principal ray alone by the pin hole to form an image of a light source pattern generated by the patterning light source 1. In the image pickup system utilizing only the principal ray, the light contributed to the focussing comprises only a straight line light in which an object is joined with an image surface and which crosses on the pin hole. In the optical conditions in the state as described, the behavior of a fine straight line beam from the camera 3 to the patterning light source 1 may be taken into consideration. When a fine rugged portion such as an undulation occurs on the specimen to be inspected 2, a slight inclination of the specimen surface resulting from the rugged portion is amplified by an optical lever effect of a light beam, and finally, a distortion occurs in an image-picked up light source pattern. Accordingly, a change from a regular pattern as described above is detected to enable inspection of a surface undulation of the specimen to be inspected 2.
The conventional undulation inspection apparatus is constructed as described above. The inspection capacity depends upon the shape of the light source pattern of the patterning light source 1. However, since the optimum light source pattern is decided as a result of repeated experiments, it is difficult to proceed with; 1 the standardization of hardware for the inspection apparatus and the lower price resulting therefrom. Further, since the inspection principle assumes a plane, if a three-dimensional surface applied with a design of automobiles, home electric appliances or the like, for example, is taken up as an object to be inspected, a deviation of an image pickup pattern occurs at the outset in a normal surface, rendering detection of undulation impossible. Moreover, in the actual detection of an undulation defect, a system for deciding an optimum pattern pitch with respect to a limit sample is not determined so that it is necessary to repeat "trial and error" many times and the efficiency for preparation of apparatus is very poor. Further, in a case where a flaw resulting, for example, from rolling, grinding or cutting remains on the surface of a specimen to be inspected, visibility of an image pickup pattern is extremely lowered dependent on the setting direction of the light source pattern, sometimes rendering inspection impossible. Furthermore, in a case where an undulation defect to be detected is extremely small, when a pitch of a light source pattern is made to be smaller than a predetermined level, an electronic moire phenomenon occurs to render inspection impossible.